1. Field of the Invention
This invention relates to a flowmeter for measuring the flow rate of a fluid flowing in a pipe conduit. More particularly, the invention relates to a flowmeter capable of measuring the accurate flow rate even in a portion where the state of flow is turbulent.
2. Description of the Related Art
To measure the flow rate of a fluid in a pipe, the fluid is made to run as a laminar flow or as a grown turbulent flow such that the flow rate distribution exhibits a known pattern; the flow rate at a certain point is measured; and the flow value is calculated, based on the measured value, the pattern of flow rate distribution and the cross-sectional area. In order to make a fluid in a pipe to run as a laminar flow or a grown turbulent flow, it is normally required for the pipe to have a straight portion over a length ten times the diameter of the pipe or more at a portion where a flowmeter is inserted. However, when the fluid to be measured flows in an ancillary pipe, such as conduit for air or gas, conduit for refrigerant or cooling water, or the like, pipes rarely have such a straight portion ten times longer its diameter or more, and it is difficult to obtain a laminar flow or a grown turbulent flow, which makes it difficult to detect the accurate amount of the flow with this approach.
Therefore, for measurement of a flow without a sufficient length of a straight pipe portion, there was frequently used a volumetric flowmeter, such as wet type flowmeter or diaphragm type meter, capable of detecting the flow amount, not relying on a flow rate distribution. Alternatively, there was used a correlative flowmeter combining some flowmeters to suppress the output variation caused by changes in flow rate distribution.
Although such a volumetric flowmeter does not require a straight pipe portion, it has drawbacks caused by its own structure, namely, (1) slow response, (2) large pressure loss, and (3) difficult assembly thereof into an existing pipe; and its use is possible only under a limited condition. In regard of conventional correlative flowmeters, they need a complicated signal processor for averaging outputs, calculating a correlative signal, and so forth.
The Pitot-tube type flowmeter disclosed in Japanese Patent Laid-Open No. 60-7207(1985) is an excellent correlative flowmeter. FIG. 16A is a partly cut-out, perspective view showing a structure of the Pitot-tube type flowmeter, and FIG. 16B is a cross-sectional view taken along the B--B line of FIG. 16A. The flowmeter comprises a Pitot's tube 90 including a high-pressure pipe 91 and a low-pressure pipe 92. The Pitot's tube 90 is housed in a hollow sheath 93. A high-pressure port 94 opens at the tip end of the high-pressure pipe 91 of the Pitot's tube 20, and a low-pressure port 95 opens at the tip end of the low-pressure pipe 92. The low-pressure port 95 is located on the axial line of a conduit in which the Pitot's tube 20 is inserted. The sheath 93 has a plurality of upstream ports 96 such that pressures of a fluid detected by the upstream ports 96 are averaged and applied to the high-pressure port 94 of the high-pressure pipe 91. Therefore, the flowmeter using the Pitot's tube can output with remarkably better S/N ratio than a single Pitot's tube, and stable outputs can be obtained with the flowmeter even when its straight pipe portion is only around several times its diameter.
Although the Pitot-tube type flowmeter does not require a complicated signal processor because of the averaging operation inherent to its structure, it still involves drawbacks inherent to the structure of the Pitot-tube type flowmeter, namely, (1) narrow detectable range of flow rates, (2) slow response, (3) high cost, and (4) the need for an expensive differential oscillator for obtaining an electric output signal.
It is therefore an object of the invention to provide a flowmeter that can be inserted in existing pipe conduits in which the flow is turbulent, has wide detectable range of flow rates and quick response, can be fabricated at low cost, and causes only small pressure loss in fluid.